The present invention relates to compression and decompression of projection data acquired for computed tomography (CT), particularly to adapting attenuation of the projection data samples to achieve a desired number of bits per sample in the compressed data.
In a CT imaging system, multiple x-ray radiographic views of an object produce sets of projection data. Each line of projection data represents an integration of density values of internal structures within a plane, or slice, of the object. From the multiple sets of projection data, the CT imaging system produces two-dimensional (2D) cross-sectional images and three-dimensional (3D) images of the internal structures of the object. The images are obtained through application well-known image reconstruction algorithms to the sets of projection data. The techniques that reconstruct cross-sectional images or three-dimensional images from multiple sets of projection data are broadly referred to as “tomography”. Performing the image reconstruction using a programmable processor-based device is broadly referred to as computed (computerized or computer-assisted) tomography. In a typical application, a source of x-ray radiation projects x-rays through an object onto an x-ray sensor (or detector) array. The x-ray sensor outputs are digitized to form a set of projection data. The set of projection data can be one-dimensional or two-dimensional depending on the geometry of the detector array. Relative movement between one or more of the object, the x-ray source and the x-ray sensor array provides multiple views having different perspectives. An image of a slice through the object, or a cross-sectional image, can be approximated by the use of mathematical transforms of the multiple views. In certain applications, the cross-sectional images may be combined to form a 3D image of the object that may be otherwise unobservable.
A well-known application of x-ray CT is in medical CT scanners for non-invasive imaging of a human body. In medical CT scanners, multiple views are obtained by rotating the x-ray source and detector array using a gantry and transferring the projection data across the slip ring. Modern CT scanners (as of 2008) digitize tens of thousands of x-ray sensor outputs in the range of one to ten kilosamples per second (ksamp/sec) with each digital sample having 16 to 24 bits per sample, resulting in an aggregate data transfer bandwidth of many gigabits per second (Gbps) across the slip ring. The projection data must also be stored or buffered in real time prior to image reconstruction. The image reconstruction process is typically 10 to 20 times slower than the data acquisition process, creating the need for storage. Typical storage subsystems include redundant arrays of independent disk (RAID) drives. As data transfer rates across the slip ring increase, the storage capacity and throughput of the RAID subsystem must also increase. As the industry strives for increased spatial and temporal resolution and increased numbers of x-ray sensors, the bandwidth demand for data transfer and data storage subsystems will soon surpass 10 Gbps.
Another application of x-ray CT is in automated inspection of industrial products. For example, cross-sectional images reconstructed from x-ray projection data is used in quality control inspection systems for manufactured products including as electronic devices, such as printed circuit boards. Tomography can be used to reconstruct images of one or more planes, or cross-sections, of an object under study in order to evaluate the quality of the object. The x-ray CT system acquires sets of projection data at various location and views with respect to the object of interest. The system architectures for industrial inspection systems differ from medical CT scanners. However, like medical CT systems, large volumes of projection data require data transfer and storage. For automated inspection systems, higher throughput of the objects under test is desirable because it reduces the cost of the product being tested. A higher throughput increases the bandwidth demands for data transfer and data storage. Another example of automated inspection using CT scanning techniques is automatic baggage screening systems.
The large volumes of projection data acquired by a data acquisition subsystem of a CT system create a burden on system resources for data transfer and data storage. Limitations in data transfer bandwidth delays the availability of projection data for the reconstruction and display of an image of the object being scanned. Compressing the projection data prior to data transfer followed by decompression before image reconstruction processing reduces the burden on system resources for data transfer and storage. The benefits of compression include reducing latency between data acquisition and image display, increasing the volume of data transferred over a communication channel having limited bandwidth, and providing compressed projection data for storage and transmission over a network for later access and image reconstruction. Since compression allows the system resources to accommodate more projection data, the image resolution can be improved and/or a larger region of the object can be scanned. The availability of computing resources to implement compression operations is also a constraint in CT systems. It is desirable that the compression operations have low computational complexity and can operate in real time to minimize the impact on computing resources.
In computed tomography, there are two domains of image-related data, the Radon transform domain and the spatial domain. The projection data, or sinogram data, are in the Radon transform domain, also referred to as the projection domain or sinogram domain. The projection data can be 2D in the situation where projection data are obtained for one slice of the object or resulting from a linear array of x-ray sensors. The projection data can be 3D in the situation where projection data are obtained for more than one slice of the object or resulting from a two-dimensional array of x-ray sensors. The 2D cross-sectional images reconstructed from the projection data are in the 2D spatial domain. A three-dimensional image reconstructed from the multiple cross-sectional images is in the 3D spatial domain. The Radon transform is the mathematical transform that underlies the relationship between the projection data in the Radon transform domain and the spatial domain image reconstructed from the projection data. Applying a compression algorithm to the projection data in the Radon transform domain will not produce the same results as applying the same algorithm to the reconstructed image in the spatial domain because of the mathematical relationship between the projection data and the reconstructed image.
Image compression techniques, for example JPEG image compression, are typically applied to spatial domain image data, for example photographic images. Spatial domain image compression techniques are also applied to reconstructed images in computed tomography for efficient image storage or transmission of the spatial domain image. An approach to achieve additional compression in the spatial domain image is to identify regions of interest in the image and apply lossless compression to the regions of interest and lossy compression to areas outside the region of interest. Examples of this approach are described in the article entitled, “Segmentation-based CT Image Compression” by Thammineni et al. in the Proceedings of SPIE, Vol. 5371, pp. 160-169, 2004, and in the conference paper entitled, “CT Image compression with Level of Interest,” by Hashimoto et al., IEEE 2004 International Conference on Image Processing, pp. 3185-88.
For the projection, or sinogram, domain, compression and decompression of projection data are applied prior to reconstruction of an image in the spatial domain. Some approaches to compression of projection data apply a JPEG image compression method in the projection domain. An example of this approach is described by Bae et al. in U.S. Pat. No. 7,327,866 entitled, “Method and Apparatus for Compressing Computed Tomography Raw Projection Data,” issued Feb. 5, 2008. This approach applies lossless or lossy compression to the projection data. An approach to compress the projection data that falls within the boundaries of object being scanned is described by Nishide et al. in the Japanese published patent application entitled, “X-Ray CT Apparatus, System and Projection Data Compressing/Restoring Method”, Kokai (unexamined) Patent Publication Number 2003-290216 (P2003-290216A), published on Oct. 14, 2003. This approach separates the projection data into air information regions, where the x-rays have traversed an empty region, and subject information regions, where the x-rays have traversed the object or patient. Different compression methods are applied to the air information region and the subject information region or the air information region may be deleted.
Disadvantages of the above approaches to compression of the projection data include the following. The bit rate of the compressed data can vary unpredictably in the above techniques because the regions of interest defined and lossless compression are data dependent. Since the bandwidth of the compressed data varies over time, an interface such as a FIFO is required to support the varying data rates. A FIFO interface is more complicated than a fixed-rate interface, since it requires additional control signals (half full, almost full, almost empty, etc.). It would be advantageous to achieve a compressed data the bit rate that falls within a desired range. A lossy fixed-rate compression mode allows control of the bandwidth of compressed data. The compressed data can then be transferred across an interface to a storage medium at a fixed data rate. The fixed data rate simplifies the interface for transfer of the compressed data and minimizes the FIFO depth. Another disadvantage of the above approaches is computational complexity, depending on which compression method is applied. It would be advantageous to reduce the computational complexity to decrease the burden on system resources and allow real time compression of the projection data.
In the commonly owned and co-pending U.S. patent application Ser. No. 11/949670 (the '670 application), entitled “Compression and Decompression of Computed Tomography Data”, filed on Dec. 3, 2007, describes compressing projection data and decompressing the compressed projection data prior to image reconstruction. The '670 application teaches classifying the projection data samples into subsets based on their significance. The compression operations applied to the subsets depend on the significance of the projection data samples.
In this discussion, “real time” means a rate that is at least as fast as the sample rate of a digital signal. The term “real time” can be used to describe rates for processing, transfer and storage of the digital signal. The sample rate is the rate at which an analog to digital converter (ADC) forms samples of a digital signal during conversion of an analog signal. When converting a digital signal to an analog signal, the sample rate is the rate at which the digital to analog converter (DAC) forms the analog signal from the samples of the digital signal. The bit rate of an uncompressed sampled, or digital, signal is the number of bits per sample multiplied by the sample rate. The compression ratio is the ratio of the bit rate of the original signal samples to the bit rate of the compressed samples. For this application, real time refers to the rate at which the ADC forms the digital samples of projection data from the output signal of the x-ray sensor.
This description refers to lossless and lossy compression. In lossless compression, the decompressed samples have identical values to the original samples. If lossless compression does not give adequate reductions in the bit rate of the compressed samples, then lossy compression may be necessary to provide sufficient reduction of the bit rate. In lossy compression, the decompressed samples are similar, but not identical to, the original samples. Lossy compression creates a tradeoff between the bit rate of the compressed samples and the distortion in the decompressed samples.